For the past 45 minutes I've been able to hear what sounds very much like a vibrator being used. It starts up buzzing for about 15 minutes, then stops for a bit, then starts again. 
I can't think what on earth else it might be.
I'm so glad NDN are getting their rocks off but it's late, and I am 7 months pregnant and struggle to sleep as it is without having to listen to buzzing half the night. I've never heard it before and oddly I can't hear any other (accompanying) sounds!
AIBU to bang on the wall and spoil their fun?!